Barrier-free contacting of ceramic bodies



Nov. 14, 1967 R. SCHOFER ET AL 3,352,713

BARRIER-FREE CONTACTING OF CERAMIC BODIES Filed Feb. 15, 1963' .77 5 6 Z5 Z0 Z4 United States Patent 3,352,713 BARRIER-FREE CONTACTING 0F CERAMIC BODIES Rudolf Schofer and Erich Fennel, Munich, Germany, as-

signors to Siemens Aktiengesellschaft, a corporation of Germany Filed Feb. 13, 1963, Ser. No. 258,675 Claims priority, application Germany, Feb. 16, 1962, S 78,061 2 Claims. (Cl. 117212) The invention disclosed herein is concerned with barrier-free contacting of ceramic bodies such as thermistors.

US. Patent No. 3,027,529, dated March 27, 1962, describes a ceramic resistor with high temperature coetficient of its entire resistance value, such resistor being made of doped ferroelectric crystallite particles which are sintered together, wherein the contacts are provided on the resistor material, especially vaporized thereon, substantially barrier-free, so as to obtain a negligibly low voltage dependence of the total resistance thereof.

In order to obtain barrier-free contacts, the above noted prior patent also contemplates pretreating with a glow discharge the points of the ceramic thermistor body, which are to be provided with contacts. Upon the points thus pretreated is vaporized a common metal, for example, aluminum or zinc. Upon the resulting metal layer may be provided, chemically or galvanically, a further layer of well solderable metal, for example, copper or silver.

It was found that the method disclosed in the prior patent provides generally for a sufficient voltage independence of the resistance value, but that such voltage independence is insufficient to meet particularly high requirements. Extensive experiments were conducted with the object of ascertaining the reasons for the slight residual voltage dependence occurring in connection with such contacting procedure. The results of these experiments showed that the conductivity of the intermediate layer was not high enough so as to exclude the barrier effect between the ceramic material and the material of the contacts. It is assumed that such insufiiciently high conductivity is due to a reduction of the doping conductivity owing to the possibility of the effect of oxygen on the ceramic material.

In order to remedy this situation, the present invention proposes to carry out without interruption, in a negative pressure apparatus, the steps required for the production of a barrier-free contacting, namely, (a) the glow-discharge treatment of the ceramic body at the points provided for the contacting, (b) the vaporizing on such points of a first layer of common metal, for example, aluminum or zinc, and (-c) the vaporizing upon the first layer of a second layer of a well solderable metal, for example, copper or silver. This means, that the preparatory glow discharge treatment and the vaporizing operations are to be carried out so that an ingress of substances or gases which can act on the ceramic material in oxidizing sense, for example, ingress of air, is to be prevented at least until the second layer of the well solderable metal is completely disposed upon the first layer. In order to secure good solderability of the ceramic thermistor, provided with the metal layers, even after prolonged storage thereof, it is advisable to cover the well solderable second metal layer, likewise without interruption of the contacting operations, with a third layer of tin which is vaporized thereon. The layer of tin is advantageously thicker than the other metal layers, having a thickness which corresponds at least to the combined thickness of the first and second metal layers.

The invention will now be described with reference to the accompanying drawing, wherein FIG. 1 shows in more or less schematic part sectional 3,352,713 Patented Nov. 14, 1967 view an example of a device for effecting the glow discharge pretreatment and for providing the layers of contact metal at the end of a rod-like ceramic body; and

FIG. 2 shows on a greatly enlarged scale part of a rod-like ceramic body provided with contact metal at the end thereof.

The apparatus shown in FIG. 1 comprises a vacuumtight receptacle or vessel which may be of desired configuration, for example, square or rectangular in cross section, having side walls 15 and 16, an end wall 17, and a bottom wall 18. The receptacle is illustrated with the top plate or wall thereof removed so as to show the interior parts. The parts are not drawn to scale but merely schematically indicated to show the cooperation and function thereof. Numeral 19 indicates a circular plate which is in suitable manner disposed vacuum-tight and rotatable and forms part of an end wall at the end opposite the end wall 17. Numeral 20 indicates a centrally disposed handle for rotating the plate 19. The conduit V.P. extends to a vac uum pump.

The plate 19 carries, in the manner of a turret, a nozzle 21 and displaced therefrom by an electrode 22, these parts being radially spaced from the central axis of rotation of the plate 19 and being mounted on the plate 19 in vacuumtight manner, the pin carrying the electrode 22 being disposed in insulated vacuumtight manner with respect to the turret plate 19. Extending outwardly from the nozzle 21 is a conduit 23 having three valved inlets 24, 25 and 26 for selectively injecting in vaporized condition, aluminum or zinc (inlet 24), silver or copper (inlet 25) and tin (inlet 26), for ejection from the nozzle.

It is of course understood that there may be provided three separate nozzles, each with an individual inlet. The valves in the respective inlets may be controlled by suitable timing means, if desired, so as to control the durationand therewith the amount of vaporized metal injected at a given pressure. If desired, suitable heating means may likewise be provided in connection with the respective inlets.

Interiorly of the vacuum vessel is disposed a frame-like insert comprising end plates 27, 28 formed of insulating material and a metal tube 29 the axis of which is radially spaced from the center by an amount corresponding to the off-center spacing of the nozzle 21 and electrode 22, respectively. Numerals 27' and 28' indicate holes to provide communication between the chambers formed by the disks 27 and 28. The rod-like ceramic body 11 which is to be provided with contact metal at an end face thereof, is placed into the metal tube 29' its ends being flush with the planes of the insulating disks 27, 28, respectively.

In the position in which the parts are illustrated in FIG. 2, the turret plate 19 has been rotated so as to dispose the electrode 22 opposite the right hand end of the rodlike body 11, relatively closely spaced therefrom. The electrode 22 and the metal tube 29 are connected in a circuit including a current source 30 and a switch 31. Upon closing the switch 31, current will flow from the electrode 22 to the ceramic body 11 and thence through the metal tube 29 back to the source 30, producing a glow discharge which affects the corresponding end face of the body 11. The contact 31 may be part of a suitable timing device so as to control the duration of the glow discharge as desired. This effects the first step of the method.

The turret plate 19 is thereupon rotated to move the electrode 22 away from the end face of the ceramic body 11, which has been subjected to the preparatory glow discharge treatment, and to dispose the nozzle 21 opposite such end face. Suitable means may be provided for locking the plate 19 in adjusted angular position. There is now injected through the inlet 24 a common metal, for example Al or Zn, in vaporized condition, so as to form the first metal layer on the prepared end face of the body 11. The second metal layer of Ag or Cu and thereafter the third layer of Sn (if desired) are thereupon in similar manner formed by injecting the corresponding metal in vaporized condition through the conduits 25 and 26, respectively.

The frame formed by the insulating disks 27, 28 and the metal tube 29, operates as a mask or shield to confine the deposit of metal to the respective end face of the rodlike body 11, the metal tube 29 also functioning as a current lead for the body 11.

In case contact metal is also to be provided on the other (left) end face of the body 11, the receptacle is opened, the body 11 is removed from the tube 29 and replaced therein in reversed position, whereupon the steps including the glow discharge preparatory treatment and the vaporizing of metal are repeated as described, each step following the preceding step in the vacuum atmosphere.

The device may also be constructed by duplicating on the left hand end the parts shown at the right hand end.

It will be appreciated that the device may be modified by appropriate disposition of electrode means and nozzle means, so as to provide metal contact layers on more than one point of the face of a ceramic body.

In FIG. 2, numeral 11 indicates part of a rodlike body and numerals 12, 13 and 14 indicate three met-a1 contact layers. The thickness of the layer 14 corresponds at least to the combined thickness of the layers 12 and 13.

It may be mentioned in conclusion, that the device may be modified by disposing the nozzle or nozzles stationary and in vacuumtight manner at one end wall of the vacuum vessel, while mounting the frame or holder for the body to be treated rotatable and in vacuumtight manner, with suitable means for rotating it so as to selectively dispose a point or points to be metallized, successively opposite the electrode means and there-after opposite the nozzle means through which the desired metal is successively injected for vaporization on the respective point or points. Suitable means may in such case be likewise provided for '4. locking the rotatable parts in any angularly adjusted position. The parts are thus again disposed for relative displacement as in the illustrated embodiment.

The invention is not inherently limited to the metallizing of rodlike bodies but may likewise be applied for metallizing a point or points on disklike or platelike bodies having a surface extending generally in a radial plane.

Changes may be made within the scope and spirit of the appended claims which define what is believed to be new and desired to have protected by Letters Patent.

We claim:

1. A method of contacting in barrier-free manner ceramic thermistors made of ferroelectric basic material, comprising subjecting .a ceramic body, which is to be contacted, to negative, less than ambient pressure, and subjecting said body, while it is under negative pressure, successively and without interruption, to the following steps, namely (a) treating with a glow discharge areas of the body which are intended for the contacting; (b) vaporizing on said areas a first layer of a common metal selected from the group consisting of aluminum and zinc; and (c) vaporizing upon the first layer a second layer of a well solderable metal selected from the group consisting of copper and silver.

2. A method according to claim 1, comprising the step of vaporizing upon the second layer a third layer of tin, directly subsequent to the application of the second layer, while said body is still under negative pressure, the thickness of said third layer corresponding at least to the combined thickness of the first and second layers.

References Cited UNITED STATES PATENTS I 3,027,529 3/1962 Schofer 252--512 FOREIGN PATENTS 952,737 11/1956 Germany.

WILLIAM L. JARVIS, Primary Examiner.

ALFRED L. LEAVITT, Examiner. 

1. A METHOD OF CONTACTING IN BARRIER-FREE MANNER CERAMIC THERMISTORS MADE OF FERROELECTRIC BASIC MATERIAL, COMPRISING SUBJECTING A CERAMIC BODY, WHICH IS TO BE CONTACTED. TO NEGATIVE, LESS THAN AMBIENT PRESSURE, AND SUBJECTING SAID BODY, WHILE IT IS UNDER NEGATIVE PRESSURE, SUCCESSIVELY AND WITHOUT INTERRUPTION, TO THE FOLLOWING STEPS, NAMELY (A) TREATING WITH A GLOW DISCHARGE AREAS OF THE BODY WHICH ARE INTENDED FOR THE CONTACTING; (B) VAPORIZING ON SAID AREAS A FIRST LAYER OF A COMMON METAL SELECTED FROM THE GROUP CONSISTING OF ALUMINUM AND ZINC; AND (C) VAPORIZING UPON THE FIRST LAYER A SECOND LAYER OF A WELL SOLDERABLE METAL SELECTED FROM THE GROUP CONSISTING OF COPPER AND SILVER. 